Search online for "your topic + primary sources" to discover collections. You may want to even add "LibGuide" to your search to find resources curated by other libraries.
In Google, go to Settings - Advanced Search and scroll down to Usage Rights. From there, select "Free to Use, Share or Modify" to make sure the images you search for are openly licensed.
Make sure to cite any image, video, or other form of multimedia in the credits of your video! Each citation style (MLA, APA, Chicago) all have their own ways of formatting citations for these sources, which you can find through a Google Search online or by contacting a librarian.
A lot of what you find online is copyrighted. That means, technically, you would have to go through a long process of seeking out permission (and maybe even paying a fee) if you wanted to use it legally for your own project.
Creative Commons is different. People who create things can choose a Creative Commons license for their work, which means you're legally allowed to use it for free as long as you follow a few simple rules (which vary depending on the license they pick).
Using images and video that is licensed under Creative Commons (CC BY) or Public Domain is your best bet to avoid copyright issues!